1. Field of the Invention
The present invention relates to a method for manufacturing light emitting diode (LED), especially to a method that manufactures the LED by wet etching.
2. Description of Related Art
In the periodic table of the chemical elements, elements of group III and group V are semiconductor materials used as main streams of light sources and photodetector materials. The LED industries in our country already have an excellent foundation for the industry development and the production value of LED has become the world's second largest producer. Thus the required amount of GaN (gallium nitride) blue/green light emitting diode is quite high.
Starting early in 1972, Pankove, as one member of stuff in Radio Corporation of America (RCA) lab, had demonstrated GaN blue light emitting diodes (with metal-insulator-semiconductor structure) successfully. Because the p-type GaN was unable to be made at that time, the LED with a pn junction was still a harder goal to obtain.
In 1981, Japanese scientist Akasaki, professor of Nagoya university, solved the p-type doping dilemma, achieving conducting material with annealed Mg-doped GaN.
In Mg-doped p-type GaN, the doping magnasium is from dicyclopentyl magnasium of organic metal. Then the grown film was irradiated by low energy electron beam for activating the magnesium atoms so as to get p-type Gan layer. By the aluminum nitride used as buffer layer, the p-type GaN layer is grown on sapphire substrate. Thus a first p-n junction blue GaN LED with light intensity of 10 microcandela (mcd) is obtained.
The Dr. Nakamura of nichia company started to work with gasllium nitride in 1989. He modified a conventional commercial Metalorganic Chemical Vapor Deposition (MOCVD) reactor and succeeded in making a two-flow MOCVD reactor to grow high quality GaN crystals. GaN films grown at low temperature are used as buffer layers while dicyclopentyl magnasium is used as a source of p-type doping. Instead of low energy electron beam irradiation of Akasaki, the frown Mg-doped GaN film is heated directly. On March 1991, a first p-n homojunction LED was invented. Then Indium gallium nitride (InGaN) film was grown successfully and high intensity, Double Heterojunction (DH) InGaN LED is obtained on December 1992. Next single-quantum-well LED and multiple-quantum-well LED were tried to grow while AlGaN or GaN were used as blocking layers. In 1994 and 1995, blue LED and green LED with 12 cd light intensity were made. In 1996, a large amount of blue LED was sold.
It is learned from researches of Dr. Nakamura that successfully developed buffer layers, p-type layers, InGaN active layers, and ohmic contact techniques have made a great progress in commercialization of the blue LED. The LED structure evolves from homojunction, heterojunction (even double heterojunction), and finally into single-quantum-well structure as well as multiple-quantum-well structure.
Since the commercialization of LED in 1960, LED has been applied to our daily essentials such as home appliances, indicators of various instruments or light sources due to the features of good shock resistance, long lifetime, low power consumption, and low heat generation. In recent years, various colorful and high intensity LED have been developed so that the applications are expended to outdoor displays such as large-scale outdoor billboards and traffic signs. The three primary colors are red, blue and green. Thus for large-scale outdoor billboards, high intensity blue LED or green LED is indispensable.
In order to increase light emitting efficiency of LED, etching is used to increase roughness of the LED surface. The etching processes include both wet and dry etching. The dry etch process etches anisotropically. Once the epitaxial structure of LED etched by dry etching, a plurality of rectangular blocks is formed on the epitaxial structure. In concept of light extraction, side planes of these rectangular blocks are parallel to one another and this has no help for extracting the light whose incident angle is larger than total reflection angle. As to the wet etching, it is an isotropic process. By the wet etching, a plurality of pyramids are formed on the epitaxial structure. The side planes of those pyramids are not parallel to one another so that light emitting area of LED increases and the light whose incident angle is larger than total reflection angle is also extracted. Thus the light emitting efficiency of LED is improved.
Once the LED is intended to be etched with larger depth, dry etching is used more often. Now the wet etching is only applied to surface roughness of LED with small etching depth. In the present invention, the epitaxial structure of LED is etched by wet etching. By control of the conditions of wet etching, the etching depth is beyond the light emitting layer and a plurality of pyramids is formed on the epitaxial structure so as to increase light emitting directions and area of LED. Therefore, the light emitting efficiency of LED is improved.